The present invention relates to microwave oscillators. More particularly, this invention pertains to a method and an apparatus for reducing the phase or frequency noise that is observed in the output of a microwave oscillator at frequencies very close to the carrier frequency.
So-called "close-in" phase noise in microwave oscillators results from the conversion of phase fluctuations in the active device or phase detector to frequency fluctuations in the oscillator output. This noise is known to result from feedback mechanisms that are inherent in oscillator operation.
Both the transistors used in oscillators with direct r.f. feedback and the mixers used in stabilized local oscillator (STALO) configurations are characterized by 1/f spectral density for phase fluctuations. (The oscillation loop converts 1/f phase noise into 1/f frequency noise that is mathematically equivalent to 1/f.sup.3 phase noise.) This 1/f noise dominates the oscillator's close-in (f less than approximately 10 kHz) phase noise performance. The magnitude of the 1/f phase noise can be reduced by reducing the 1/f phase noise of the active device or by increasing the Q of the stabilizing resonator.
A number of technologies that rely upon low frequency oscillators are currently employed to reduce microwave phase noise at all offset frequencies. High stability can be attained for small offsets (f less than 100 Hz) by locking the oscillator to a harmonic of a low frequency (5 MHz) quartz bulk acoustic wave (BAW) crystal oscillator. Further stabilization may be provided by a SAW oscillator operating at about 500 MHz for offset frequencies in the 100 Hz&lt;f&lt;10 kHz range. The microwave oscillator itself typically provides the best possible stability only for the relatively large offset frequencies (f greater than or equal to 10 kHz). The improved phase noise performance that can be obtained from BAW and surface acoustic wave (SAW) oscillators follows from the fact that, while BAW and SAW quartz crystals exhibit Q values of 10.sup.6 and 10.sup.5, respectively, microwave cavities and dielectric resonators are limited to about 1-3.times.10.sup.4. Similarly, active devices available at the lower operating frequencies of the quartz devices exhibit 1/f phase noise of -140 db/Hz and less at an offset frequency of f=1 Hz while the best X-band amplifiers have a larger noise of -120 db.
Due to the shortcomings of the prior art arrangements for stabilizing microwave oscillators at close-in frequency fluctuations, extremely high-Q resonators are often required to obtain satisfactory performance. Accordingly, the oscillator expense can be effectively dictated by the type of resonator "demanded". For example, if a microwave sapphire resonator (Q of 100,000 or more) is required by the noise constraints placed upon a microwave oscillator as opposed to either a cavity resonator (Q of approximately 10-30,000) or a dielectric resonator oscillator (DRO; Q up to 10,000), a one hundred (100) times cost increment in the resulting device can occur at present-day prices. Thus active device noise effectively dictates the cost of present day microwave oscillators through its effect upon resonator performance.